This application is based on and claims priority from Japanese Applications Nos. Hei. 3-336414, Hei. 4-41138 and Hei. 4-217764 filed Oct. 21, 1991, Feb. 27, 1992 and Aug. 17, 1992, respectively, the disclosures of which are incorporated by reference herein.
The present invention relates to a zoom lens system of a telephoto type that has a zoom ratio of about 3 and which is particularly suitable for use with single-lens reflex cameras.
Conventional zoom lens systems of a telephoto type are available as a four-group composition that comprises the first and fourth lens groups that are fixed during zooming, the second lens group that is moved for zooming along the optical axis, and the third lens group that is moved along the optical axis to correct any shift in the focal position due to zooming. In this case, the overall lens length can be held constant.
Another type of the four-group composition is such that the four lens groups are respectively movable along the optical axis during zooming in this case, variable magnification and other functions can be shared by the respective lens groups.
In order to further reduce the overall lens length of the type that has the first and fourth lens groups fixed during zooming, the refractive powers of the individual lens groups must be increased but this introduces difficulty in achieving effective correction of aberrations, thus making it difficult to realize a compact overall system.
With the second type which permits the four lens groups to be respectively moved along the optical axis during zooming, one can avoid the situation in which the refractive power of any particular lens group becomes excessive; however, the amount of movement of individual lens groups tends to increase. It is possible to reduce the overall lens length at the wide-angle end but, on the other hand, the overall lens length at the telephoto end will increase.
Stated more specifically, the amount of movement of individual lens groups tends to increase, causing problems with handling properties during zooming as exemplified by the front end of the lens hood making accidental contact with an object exterior to the lens system, as well as the center of gravity of the camera shifting in position by such a great amount as to cause difficulty in shooting.
Further, both types of zoom lens systems have had a common problem in that it is mechanically difficult to adopt automatic focusing since the first lens group has to be moved by a large amount.
An object, therefore, of the present invention is to provide a zoom lens system that is fairly compact and easy to handle despite the fact that it has a constant overall lens length and which requires the first lens group to be moved only by a sufficiently small amount during zooming to facilitate the adoption of automatic focusing.